1. Field of the Invention
The present invention relates to the field of current sensing and limiting circuits.
2. Prior Art
Current sensing and limiting circuits are commonly used in various systems for the protection of the circuit to which the current is being provided, the circuit which is providing the current, and/or for the protection of other circuits connected thereto. While the present invention is not limited to any specific use, the applications for which the preferred embodiment of the present invention is intended is what are commonly referred to as solid state circuit breaker, and hot swap applications wherein a printed circuit board is to be plugged into a system without turning off power to the system. Successful hot swapping in systems such as computer systems may allow the replacement of a faulty board or the addition of a new board without requiring interruption of the operation of the rest of the system and without the time delay of rebooting the system. However, successful hot swapping is not automatically achieved. In particular, the circuit on a printed circuit board to be plugged into an active system may have a substantial capacitance, the charging of which during the plug-in can cause a momentary drop in the supply voltage to cause one or more errors in the rest of the system. Further, it is common to add capacitance to the power supply lines within a printed circuit board, both to limit power supply noise generated by the circuits on the board from having a substantial effect on the power supply as seen by other boards in the system, and to limit the effect of power supply noise generated elsewhere in the system from effecting that particular board. Such additional capacitance frequently is substantially greater than that of the circuits themselves.
In other situations, such as in the case of inductive loads or incandescent lamps, inrush currents typically will be many times the steady state currents for such circuits. Also, while normal operation of a circuit will have predefined, maximum current limits, extraordinary loads due to temporary or permanent circuit faults may be encountered. To avoid high current inrush problems and steady state current problems, current limiting circuits have been used wherein the in-rush of current on the sudden power-up of a printed circuit board (or other part of a system) caused by such occurrences as the plug-in of a board into an already activated circuit, and/or a fault condition, is purposely limited so that the effects on the power supply lines and power supply itself are limited. By way of example, Unitrode Corporation and Linear Technology Corporation both provide circuits for current limiting in such applications. These devices, however, do not have independently adjustable trip thresholds. The Unitrode devices provide a different trip threshold during start-up than during normal operation, but the difference is preset to be 1 amp greater during start-up than for normal operation. Linear Technology Corporation devices use an RC time constant at startup and ignore the current levels. Therefore both of these are a compromise between different considerations.